The present invention relates to a quadrupole mass spectrometer.
In a quadrupole mass spectrometer, ions generated in an ion source including various kinds of mass numbers are drawn into a quadrupole mass filter, where, among them, only ions having a certain mass number (which are referred to as object ions) can pass through. The object ions that have passed through the quadrupole mass filter are detected by an ion detector, which generates a detection signal corresponding to the number of detected ions. Most of the ions other than the object ions dissipate in the quadrupole mass filter, but energy particles including non-charged particles, such as neutrons, which are not affected by the electric field produced by the quadrupole mass filter and some charged particles other than the object ions which do not dissipate but pass through the electric field produced by the quadrupole mass filter can enter the ion detector Such energy particles constitute noise components in the detection signals. In order to improve the sensitivity of detection, it is desired to prevent these unwanted energy particles from entering the ion detector.
In conventional quadrupole mass spectrometers, various measures have been taken to avoid such non-charged particles. One is a so-called Off-Axis structure in which the ion detector is placed offset from the ion axis of the quadrupole mass filter In another structure, an ion converging lens is placed at the exit of the quadrupole mass filter, and the lens aperture is shrunk and an appropriate converging voltage is applied to the ion converging lens so that the converging efficiency for the object ions is the highest, or, in other words, the maximum amount of object ions may enter the ion detector.
Though the efficiency of introducing object ions into the ion detector is improved with those measures, noise level is not yet sufficiently suppressed, and an increase in the S/N ratio of the detection signal is still greatly needed.
Since conventional methods cannot cope with this need, the present invention provides a quite different method and apparatus for decreasing noise in the detection signal, and thus increasing its S/N ratio, which leads to an improved analyzing sensitivity of a quadrupole mass spectrometer.
In some quadrupole mass spectrometers, an ion converging lens is placed at the exit of the quadrupole mass filter to converge object ions and make as many of them as possible enter the ion detector. With long experience in designing and a profound insight into the quadrupole mass spectrometer, the inventor experimented and studied the relationship between converging voltage applied to the ion converging lens, the intensity of object ions and the intensity of noise detected by the ion detector. Through experiment and study, the inventor found that the converging efficiency of the object ions reaches its greatest efficiency at a certain maximum converging voltage. It is also found that the converging efficiency drops as the converging voltage increases from the maximum converging voltage, and the noise intensity decreases faster than the converging efficiency of the object ions as the converging voltage increases from the maximum converging voltage. That is, with respect to the overall S/N ratio, the optimal state is achieved when another certain converging voltage, or an optimal converging voltage, which is greater than the maximum converging voltage is applied to the ion converging lens.
Thus a quadrupole mass spectrometer according to the present invention includes:
an ion source for generating ions;
a quadrupole mass filter for selectively passing object ions having a predetermined mass number among the ions from the ion source;
an ion detector for detecting the object ions;
an ion converging lens placed between the quadrupole mass filter and the ion detector; and
a voltage source for applying voltage to the ion converging lens, wherein the voltage has a polarity opposite to that of the object ions and the absolute value of the voltage is larger than that of the voltage at which the converging efficiency of the object ions is the largest.
The quadrupole mass spectrometer of the present invention and its variations are described in detail in the following description accompanied by the drawings.